1. Field of the Invention
This invention relates to switched reluctance motor controls, and, more particularly, to a method and a circuit for controlling multiple phases of a switched reluctance motor by sensing a characteristic of a current in a single phase of the motor.
2. Disclosure of Related Art
A conventional switched reluctance motor (SRM) includes a stator having a plurality of pairs of diametrically opposed stator poles and a rotor having a plurality of pairs of diametrically opposed rotor poles. Windings or coils are typically disposed about the stator poles and the windings around any two diametrically opposed stator poles may be connected in series or in parallel to define one motor phase of the multiphase SRM. The windings associated with a motor phase may be referred to as a phase coil. By generating current through the phase coil, magnetic fields are established about the stator poles and a torque is produced that attracts a pair of rotor poles into alignment with the stator poles. The current in the phase coils is generated in a predetermined sequence in order to produce a constant torque on the rotor. The period during which current is provided to the phase coil--and the rotor poles are brought into alignment with the stator poles--is known as the "active stage" or conduction interval of the motor phase. At a certain point--either as the rotor poles become aligned with the stator poles or at some point prior thereto--it becomes desirable to commutate the current in the phase coil to prevent a negative or braking torque from acting on the rotor poles. Once this "commutation point" is reached, current is no longer generated in the phase coil and the current is allowed to dissipate from the phase coil. The period during which current is allowed to dissipate from the phase coil is known as the "inactive stage" of the motor phase.
In order to maintain a relatively constant torque on the rotor--and to thereby optimize motor efficiency--it is important to maintain an "in-phase" relationship between the position of the rotor and the active stage or conduction interval of each motor phase. In other words, it is important that the conduction interval be initiated, controlled, and commutated as the rotor reaches predetermined rotational positions. If the conduction interval is initiated and/or commutated too early or too late with respect to the position of the rotor (i.e., the conduction interval "leads" or "lags" the rotor), a constant torque on the rotor will not be maintained and the motor will not operate at an optimum efficiency.
Conventional switched reluctance motors have attempted to maintain an "in-phase" relationship between the conduction intervals of the motor phases and the position of the rotor by continuously sensing rotor position and adjusting the control signals that initiate and commutate the conduction intervals in response thereto. Conventional motors have employed a variety of "direct" and "indirect" methods and means for sensing rotor position. Conventional direct sensing means have included Hall-effect sensors and optical sensors mounted directly on the rotor or disposed proximate thereto. These direct sensors are disadvantageous because they consume a large amount of space, are relatively expensive and are unreliable.
Conventional indirect sensing methods and circuits have included measurements of phase coil currents or flux that are indicative of rotor position. Indirect sensing methods and circuits are generally less expensive and more reliable than direct sensing methods and circuits. Conventional motor control methods and circuits, however, use indirect sensing to measure current or flux characteristics in each motor phase of the motor--resulting in several disadvantages. First, measurement of current or flux characteristics in each motor phase of the motor consumes a relatively large amount of microprocessor resources. Second, indirect sensing of rotor position is generally accomplished by generating a sensing current pulse during a period of falling inductance in each motor phase--thereby introducing a braking torque on the rotor. Measurement of current or flux characteristics in each motor phase, therefore, further reduces the efficiency and maximum speed of the motor. Further, because braking is accomplished by initiating conduction intervals during the period of falling inductance, the use of current sensing pulses in each motor phase during the same period reduces the maximum duration of the conduction intervals, thereby reducing available braking torque. Finally, because each sensing current pulse contributes to an increase in acoustic noise, the use of current sensing pulses in each motor phase results in an undesirable level of acoustic noise.
There is thus a need for a method and a circuit for controlling a switched reluctance motor that will minimize or eliminate one or more of the above-mentioned deficiencies.